The present invention relates to a rotating head helical scanning-type magnetic recording and reproducing apparatus, and, more particularly, to a magnetic recording and reproducing apparatus for video signals in which tracking control is effected using pilot signals.
In a rotating head helical scanning-type video signal recording and reproducing apparatus, typical of which is a household-use video cassette recorder (VCR), a certain synchronizing signal, apart from the video information, is recorded on a magnetic tape during recording so as to effect tracking control. During reproduction, the control of the rotational phase of a capstan or the rotating head is effected by making use of the reproduced synchronizing signal, and tracking control is effected by a servo circuit so that the rotating head will correctly trace the video signal recording tracks of the magnetic tape.
As for the synchronizing signal, a pulse signal which is generally called a control signal and has a 1/2 cycle of the vertical synchronizing signal is used, and a system whereby this control signal is used after it is written at one edge of the tape by a fixed recording and reproducing head has been employed. However, since the position at which the control signal is recorded in accordance with this system and the video track are distanced from each other, if reproduction is carried out using an apparatus different from the one employed in recording, or due to change with age even if recording and reproduction are carried out by the same apparatus, there are cases where a tracking error occurs. For this reason, there has been a problem in that it becomes necessary to provide a manual tracking adjustment means.
Consequently, in recent years, a servo tracking system (called the pilot system for short) has been devised in which four kinds of pilot signals f.sub.1, f.sub.2, f.sub.3, and f.sub.4 having different frequencies are recorded on the video track as synchronizing signals in a superimposed manner to enable automatic tracking.
As an example of such a system, there is one disclosed in U.S. Pat. No. 4,297,733. In this system, four kinds of pilot signals of f.sub.1 (6.5 f.sub.H), f.sub.2 (7.5 f.sub.H), f.sub.3 (10.5 f.sub.H), and f.sub.4 (9.5 f.sub.) (where f.sub.H is the horizontal synchronization frequency) are in that order written in tracks for each field, and, during reproduction, not only the pilot signal in the scanning track of the rotating head, but also those in both adjacent tracks are read simultaneously. A differential signal of pilot signals in both adjacent tracks is produced as an analog signal using a differential amplifier, and a tracking error signal is produced on the basis of this differential signal, thereby effecting tracking control of the magnetic tape.
This pilot system permits an automatic tracking function, but since the tracking control is effected on the basis of the magnitude of the differential signal, there have been the following drawbacks:
(1) A tracking error occurs owing to the drift of constants and characteristics of components in the servo circuit caused by temperature change and the like, thereby causing a deterioration in the signal-to-noise ratio of pictures.
(2) If the loop gain of the servo system undergoes a major change due to the reproduction level of reproducing pilot signals with a resultant decline in the reproduction level, there is the possibility of the stability of the system becoming deteriorated, thereby resulting in the misengagement of the servo system and the like.
In particular, in cases where the track width is narrowed to enhance the recording density of signals and in the case of a magnetic recording and reproducing apparatus having two kinds of recording tape speeds of SP (standard play) mode and LP (long play) mode (e.g., the tape speed of LP is one half that of SP), since the reproducing pilot level declines substantially, the problem caused by fluctuations in the reproducing pilot level in the aforementioned drawback (2) has been large.